This invention relates to the field of electrical delay lines wherein signal delay is achieved by propagation through a ferrimagnetic film such as a magnetic garnet and to achieving the parallel magnetic field employed with such delay lines operating in the backward volume wave operating mode.
Magnetostatic wave delay lines find increasing use in the processing of electronic signals in such applications as signal correlation, where two periodically occurring signals are compared in order that properties such as signal source movement, strength change, or source velocity change be detectable. The magnetostatic wave delay line is additionally found useful for phasing signals in a multiply arrayed antenna system operated in the higher frequencies used for radar and other line-of-sight communication.
Applications of the magnetostatic delay line are limited by a fundamental property of magnetostatic device technology concerning time delay characteristics. According to this limitation the delay vs. frequency characteristics of a magnetostatic propagating wave is neither constant nor linearly varying with respect to frequency. Such non-linearity is known as the dispersive behavior of a magnetostatic delay line; that is, the phase velocity and the signal group velocity in a magnetostatic delay line are found to be unequal.
A promising arrangement for linearizing behavior of a magnetostatic delay line is found in the technique of dispersion cancellation. According to this technique, two magnetostatic delay lines of for example, yttrium iron garnet (YIG) are employed in cascade or tandem, with one YIG element biased using a magnetic field which is parallel to the direction of signal propagation, while the other is biased with a magnetic field perpendicular to the direction of signal propagation. In this arrangement, the parallel magnetic field generates a magnetostatic backward volume wave (MSBVW) while the perpendicular magnetic field generates either a magnetostatic surface wave (MSSW) or a magnetostatic forward volume wave (MSFVW). The time delay vs. frequency characteristics of the MSBVW propagating wave and that of either of the latter two types of propagations are oppositely sloped in a YIG element to a degree permitting cancellation of the dispersive characteristics and allowing a delay line employing the combination of propagation modes to have a relatively constant delay over at least a limited frequency range. A problem in achieving this desirable dispersion cancellation arrangement is concerned with the need to produce a parallel magnetic field suitable for achieving the magnetostatic backward volume wave (MSBVW) mode of operation; such a field must be long, narrow, and uniform over the body of the MSBVW element.
The patent prior art includes several examples of magnetostatic delay line devices and arrangements for achieving the desired magnetic field for such devices; these examples include the patent of W. S. Elliott, U.S. Pat. No. 3,480,888, which describes a YIG element used with a biasing magnetic field to embody an electronically tuned wave filter whose passband center frequencies can be varied by changing the strength of the biasing DC magnetic field. The Elliott invention contemplates the filter element to be in the form of a sphere and for this sphere to be located in a solenoidal electromagnet. The solenoidal electromagnet has a degree of uniform magnetic field intensity over its internal cross-section and changes in this field intensity are used in the Elliott invention to change the center frequency of the filter element.
An example of a YIG element used in combination with a biasing magnetic field to achieve a magnetostatic delay line is found in the patent of C. F. Vasile, U.S. Pat. No. 3,530,409, wherein a pair of permanent magnets are used to bias a slab, bar or cylindrical rod into magnetic saturation in order to achieve a changed spin wave propagation. The Vasile invention contemplates the use of specially tailored magnetic fields in order to achieve desired characteristics from the YIG element, the desired magnetic field being achieved through the use of a second length of poly-crystal YIG material placed adjacent the single-crystal delay line element in order to achieve an increasing bias field in the delay line element.
An example of prior art magnetic field shaping for use with a magnetostatic delay line is found in the patent of Robert A. Moore, U.S. Pat. No. 3,593,215, wherein a YIG delay line element is located between the poles of a bias field magnet. In the Moore invention, the dispersive characteristics of the magnetostatic delay line are improved and controlled by the placement of a ferrite material in intimate contact with the YIG delay line element at its ends in order that the ferrite magnetic field shaping material and the delay line radio frequency energy circuits interpenetrate. The Moore patent also refers to a published magnetic field article titled "Internal Magnetic Field Analysis and Synthesis for Prescribed Magnetostatic Delay Characteristics" appearing in the Journal of Applied Physics, Vol. 37, No. 3, pp. 983-987, March 1966. The Moore patent is concerned with the differences between a YIG element having uniform magnetic field as provided by the two magnet poles of the Moore apparatus and a YIG element subjected to a varying magnetic field through the use of ferrite elements located at the ends of the YIG element. Claim 5 of the Moore patent indicates the presence of a magentic field applied along the YIG element and a magnetic field bias circuit disposed to interpenetrate the radio frequency magnetic field circuit for shaping the applied magnetic field.
Another example of magnetostatic wave delay lines employing magnetic field variations is found in the patent of Frederick R. Morgenthaler, U.S. Pat. No. 3,895,324, which discloses the use of a YIG material doped with gallium and employed in a graduated magnetic field which accomplishes conversion between plural wave propagation modes. The Morgenthaler patent contemplates changes in the effective magnetic field by providing appropriate material graduations, as by doping the YIG material, and further contemplates variation of the wave energy delay within the YIG material by changing the internal magnetic field bias in magnitude and/or direction. The Morgenthaler patent is also concerned with the difference between YIG element internal and external bias fields and provides arrangement for varying the magnetic field through the use of a multilayer structure.
An example of a ferrimagnetic material used in combination with a permanent magnet and a current-carrying electrical conductor is found in the patent of Joseph W. Simon, U.S. Pat. No. 3,946,340, which describes a radio frequency signal phase shifter of the type commonly used in radar systems and achieves temperature compensation of a phase shifter property through the use of a permanent magnet generated bias field.
Another example of apparatus used for generating a magnetic field suitable for biasing a ferrimagnetic crystal such as YIG is found in a second patent of Morgenthaler, U.S. Pat. No. 4,093,929, wherein a ferrimagnetic crystal which is variously labeled as 2A' or 2B, is located axially between two magnetic poles or within an electrical solenoid in order that exposure to a linear longitudinal magnetic bias field be achieved. The Morgenthaler '929 patent describes a detailed mathematical procedure for pole face shaping in order that either a spatially uniform or non-uniform magnetic field encompassing the ferrimagnetic crystal be achieved. The Morgenthaler '929 patent contemplates use of the described magnetic field generating apparatus in delay line and RF filtering applications including delay lines having linear time delay vs. frequency characteristics over an extended bandwidth. The thrust of the Morgenthaler '929 patent is the suitable shaping of the magnetic source pole faces or solenoid geometry in order to achieve a desirable magnetic flux pattern.
Yet another example of ferrimagnetic YIG materials used in combination with externally magnetic fields is found in a third patent of F. R. Morgenthaler, U.S. Pat. No. 4,152,676, which is concerned with the phenomenon of localized, extremely sharp resonances in a single-crystal YIG slab subjected to highly non-uniform demagnetization fields involving a non-ellipsoidal shape. The '676 Morgenthaler patent is concerned with this local resonance in a single crystal in contrast with the uniform precession mode YIG operation wherein all portions of each magnetic element resonate in phase as a result of a uniform internal DC magnetic field. The Morgenthaler '676 patent contemplates the use of DC magnetic fields generated by an electromagnet, these fields being generally perpendicular to the YIG crystal structure, rather than lengthwise along the crystal.
Yet another example of YIG ferrimagnetic material used in combination with permanent magnets forming a bias field is found in the patent of Bernard Chiron, U.S. Pat. No. 4,152,677, which concerns a microwave isolator wherein two YIG elements are subjected to a cross-wise oriented magnetic field.